1. Field of the Invention
The present invention relates to sensors for sensing an environmental condition, and methods for making such sensors. More particularly, the present invention relates to chemical sensors that employ one or more microcantilever-based a chemical sensing elements for sensing vapor-borne chemicals (e.g., hydrogen), and methods for making and using such sensors.
2. Background Information
The following description includes information that may be useful in understanding the present invention. It is not an admission that any such information is prior art, or relevant, to the presently claimed inventions, or that any publication specifically or implicitly referenced is prior art.
Various types of sensors have been used to detect the presence of chemical or biological agents. Among these various sensor types, sensors that detect gases such as molecular hydrogen have been used to detect the presence of a chemical in a gas, such as the atmosphere, or “air”, in a building or other structure.
In the context of hydrogen sensing, or “detection”, some applications include detecting leaks in manufacturing plants that use or produce hydrogen, detecting leaks in fuel-cell-powered vehicles, or to monitor the inlet and/or exhaust gases of hydrogen or fuel-cell-powered vehicles to optimize the mixture of hydrogen and oxygen within the engine or fuel cell, as the case may be. In addition, public garages, for example, can employ such sensors to monitor for the accumulation of dangerous explosive gases such as hydrogen.
In many applications, including the fuel-cell-powered vehicle and public garage applications mentioned above, it is desirable to use low-cost, low-maintenance sensors. Thin film coated microcantilever sensors are uniquely suited to low-power, low-maintenance applications. Clean, unoxidized transition metal surfaces, such as palladium surfaces, are known to catalyze, for example, the breakdown of molecular hydrogen into atomic hydrogen, which can then diffuse into the metal. In such sensors, when the metal absorbs atomic hydrogen, it can undergo a number of physical changes that, if properly monitored, can indicate, for instance, the ambient concentration of hydrogen (or other analyte of interest, depending upon the design of the sensor and the materials used in its construction) in the environment in which the sensor is placed. Examples of such physical changes include variation of electronic properties, such as increasing resistance, as well as decreasing refractive index, and increasing volume.
The expansion of the thin film when it absorbs an analyte, for example, atomic hydrogen, is useful in the context of microelectromechanical systems (MEMS) microcantilever sensors. Such MEMS sensors generally include beams, membranes, and/or other mechanical structures on the order of 0.1 mm to 1 mm in length, integrated with circuitry that electrically measures and/or controls the motion of the structures. A thin film applied to the beam or membrane will, upon absorbing hydrogen, expand and thereby deform (i.e., stress) the structure. This deformation can be measured as a change in capacitance between the cantilever beam and a stationary baseplate. Since, the absorption of hydrogen into the thin film is fully reversible, at any given moment the sensor capacitance should ideally indicate the current hydrogen concentration.
MEMS microcantilever sensors are particularly suited for low-cost, low-power applications such as those described above. If manufactured via standard microlithography techniques, MEMS sensor arrays containing numerous individual sensors can prove extremely cost-effective. Furthermore, these types of devices can require as little as 50 microwatts when in continuous operation. Thus, thin film coated microcantilever sensors present interesting options for developing low-cost, low-power chemical sensors.
One problem with thin film coated microcantilever sensors is that, over a period of time the thin film “relaxes” back toward its original dimensions. It is suspected that oxidation of the thin film surface causes this “relaxation.” This “relaxation” makes sensors difficult to calibrate and may prevent them from responding at all to a gradual accumulation of an analyte of interest (e.g., hydrogen).
Thus, there is a need for improved bas-borne chemical sensors that accurately and reliably measure the ambient amount, most often in terms of concentration, mass, or parts per million or billion, of a particular analyte, for example, molecular hydrogen, oxygen, nitrogen, helium, propane, and natural gas. There is also a need for such sensors to be low-cost, low-power devices so that they can be employed in a variety of applications.